1. Field of the Invention
The present invention relates to a method of improving the flow properties of a wax containing crude oil or natural gas liquid containing a natural pour point depressant by adding a synthetic pour point depressant.
2. The Prior Art
Typically, crude oils and natural gas liquids contain several types of hydrocarbons, such as paraffins, naphtenes and aromatics. However, the paraffins, which are normally liquid or gaseous at reservoir conditions, have the disadvantage that at lower temperatures they tend to form a solid wax phase. In practice this gives rise to problems, such as formation of a gel with a significant gel strength, e.g., during shut down of wells, pipelines, processing plants, etc. Likewise, problems caused by wax exchangers can occur. Furthermore, the formation of a solid wax phase can lead to an increased viscosity, which means that the crude oil or natural gas liquid may become significantly more difficult to handle.
For the purpose of alleviating the above problems it has been known for long to add so-called xe2x80x9cpour point depressantsxe2x80x9d in order to lower the pour point of the crude oil or natural gas liquid, which point according to ASTM D97 or ASTM D5853 is defined as the temperature at which a crude oil or natural gas liquid at given conditions starts to solidify. Such synthetic pour point depressants may comprise a wide range of polymers and copolymers (polyacetates, polyacrylates, polymethacrylates, polyamides, etc.).
The polymers may roughly be divided into nucleators and growth arrestors depending on their function in the crude oil or natural gas liquid. However, it is a common feature thereof that they are incorporated in the solid wax phase and thereby change its structure and properties. To achieve improved efficiency, it is normal practice to add the former type of wax inhibitor at or close to the wax appearance temperature (WAT) of the crude oil or natural gas liquid, which is defined as the temperature at which the wax starts to precipitate. The latter type of wax inhibitor can be added at lower temperatures, although the efficiency hereof seems to increase if the polymer is incorporated already in the solid wax phase first formed.
In certain crude oils and natural gas liquids there are, however, also naturally occurring pour point depressants for instance in the form of asphaltenes, which are a group of heavy polar aromatic compounds having a molecular weight in the range 700-1000 g/mole. These natural pour point depressants are present in the crude oil or natural gas liquid when it is produced from the reservoir, and are normally fully dispersed in the liquid phase at initial reservoir conditions. At lower temperatures they may, however, start to flocculate, which reduces their pour point depressing efficiency. This phenomenon gives rise to the concepts upper and lower pour points, as defined in the standard ASTM D97. Flocculation of the natural pour point depressants may also be induced by changed pressure or mixing with water or other oils/condensates. Just like synthetic pour point depressants they change the structure of the wax phase formed, although in many cases their mere presence does not completely suffice for solving the problems associated with wax precipitation.
In practice during oil production, an amount of the abovementioned synthetic pour point depressants has therefore been added to crude oils and natural gas liquids at a temperature close to the WAT of the crude oil or natural gas liquid, assuming that thereby an additive effect of the natural and synthetic pour point depressants would be obtained.
In certain cases this additive effect does not appear, as the naturally occurring pour point depressants seem to be capable of counteracting and even totally neutralizing the effect of the synthetic pour point depressants.
Consequently, it is the object of the present invention to provide a method of reducing or eliminating the above problems and thereby ensure an effective improvement of the flow properties of crude oils and natural gas liquids.
The method according to the present invention is characterized in that the synthetic pour point depressant is added to the crude oil or natural gas liquid at a higher temperature than 20xc2x0 C. below the inversion temperature of the crude oil or natural gas liquid.
The inversion temperature of a crude oil or natural gas liquid is determined by the use of a modified ASTM D97 or modified ASTM D5853 standard method. These standards specify upper and lower pour points to be measured after reheating the sample to 48xc2x0 C. and 105xc2x0 C., respectively. By reheating the crude oil or natural gas liquid to a given temperature and subsequently determining the pour point of this sample and repeating this measurement at different reheating temperatures and subsequently depicting these pour points as a function of the reheating temperature, the reheating temperature at which the pour point has been reduced to its minimum value can be determined. This point is defined as the inversion temperature, and is the temperature at which the natural pour point depressants are fully active. The result will typically be within the interval 80-110xc2x0 C.
The inversion temperature definition is, however, not applicable to hydrocarbon samples, where the upper and lower pour points are identical. This phenomena is believed to be due to the lack of natural pour point depressants mainly found among the asphaltene fraction of the fluid.
When using the method according to the invention it has turned out that in situations in which addition of synthetic pour point depressants has otherwise been without result, a positive effect can be obtained. This is presumably due to the pour point depressant being added to the crude oil or natural gas liquid at a time when the natural pour point depressants are fully or at least partly dispersed. In this way it is avoided that the flocculated natural pour point depressants, which apparently form a separate, polar liquid phase, are capable of absorbing the synthetic pour point depressants by virtue of their content of polar groups.
Due to a more efficient pour point depression, it is possible to obtain a suitable pour point even by addition of small amounts of synthetic pour point depressants. Alternatively, a lower pour point can be obtained by using the usual amount of synthetic pour point depressant.
Preferably the synthetic pour point depressant should be added at a higher temperature than e.g. 10xc2x0 C. below the inversion temperature, more preferably at or above the inversion temperature. If the synthetic pour point depressant is added at a temperature 20xc2x0 C. below the inversion temperature of the crude oil or natural gas liquid, the natural pour point depressants are thus not fully dispersed, which means that the efficiency obtained is not optimal.
In practice, the best effect is obtained by injecting the synthetic pour point depressant into the well bore at a depth, where the crude oil or natural gas liquid has a temperature at or higher than the inversion temperature.
An appropriate depth for adding the synthetic pour point depressant, may be determined as the depth at which the formation temperature corresponds to the inversion temperature of the crude oil or natural gas liquid produced. Since the temperature at this point automatically will increase once the production starts, adding the depressant at this depth ensures that the temperature of the crude oil or natural gas liquid will always exceed the inversion temperature when mixed with the synthetic pour point depressant.
The required dosage of synthetic pour point depressant depends on factors such as the type of crude oil, amount of wax formed in the crude oil at different temperatures, ambient conditions etc. The optimum dose rate is normally estimated by means of laboratory measurements of pour point, viscosity, gel strength and wax deposition tendency. The two latter measurements are often carried out on laboratory coils. In practice the synthetic pour point depressant is added in an amount of 5-2000 ppm, preferably 20-500 ppm and most preferably 20-100 ppm.
A preferred synthetic pour point depressant useful in the method according to the invention comprises a chemical compound having the general formula I: 
where n=20-300, preferably m=25-100 and most preferably m=50-80, n=5-50, preferably n=5-25 and most preferably n=8-15 and R symbolises a hydrocarbon, preferably an aliphatic hydrocarbon and most preferably an aliphatic hydrocarbon consisting of 2-20 carbon atoms.
However, also other compounds may be used, such as polyacetates, polyacrylates, polymethacrylates, polyamides etc.
The invention is described in more detail in the following examples taken in conjunction with the attached drawings.